Field Application of Calcium to Reduce Phytophthora Stem Rot of Soybean, and Calcium Distribution in Plants

The effect of calcium compounds [Ca(HCOO)(2)-A and Ca(NO(3))(2)] on the incidence of Phytophthora stem rot of soybean (Glycine max) cv. Tanbakuro was investigated in the field. Disease incidence in control plants in three fields naturally infested with Phytophthora sojae ranged from 11.7 to 52.0% at 140 days after transplanting. Independent of the pathotype diversity, 4 and 10 mM of the calcium compounds applied twice (prior to transplanting and 14 days after transplanting) significantly suppressed disease incidence and delayed onset. Ca(HCOO)(2)-A (Suicaru) was more effective than calcium nitrate for reducing disease incidence. In most cases, the calcium amendments increased plant height, number of nodes and pods, and seed yields, and reduced low-quality seeds. Scanning electron microscopy with fresh samples showed increased accumulation of calcium crystals around the cambium and xylem elements of soybean plants treated with 10-mM Ca(HCOO)(2)-A and Ca(NO(3))(2). Mycelial penetration was inhibited at these sites. These results indicated that calcium-rich areas may be more resistant to invasion by P sojae, and the calcium crystals may play an important role in calcium ion storage and its availability for those tissues to maintain long-term field resistance.